2: Physiology of the Visual Cortex Flashcards
Describe the laminar organization of the lateral geniculate nucleus.
LGN has 6 layers, 3 from each eye, alternating
Outer 4 layers = PARVOCELLULAR LAYERS: receive info about COLOR vision, fine discrimination of shape
Inner 2 layers = MAGNOCELLULAR LAYERS: receive info about MOTION DETECTION, depth, and contrast
–Most input to LGN (80-90%) is from cortex to regulate input of visual information
Diagram the visual projections from the retina to the lateral geniculate nucleus.
Left visual field -> right LGN
Right visual field -> left LGN
Describe the visual deficits from lesions in the optic nerve, optic chiasm, optic tract, LGN, optic radiation, and Meyer’s loop.
Damage to optic nerve: monocular blindness
Damage to optic chiasm: lose outer parts of visual fields in both eyes (bitemporal hemianopia
Damage to optic tract, LGN, or optic radiation: lose vision from the opposite side (homonymous hemianopia)
Damage to Meyer’s loop: loss of upper half of visual field
Discuss simple cells of the visual cortex in terms of their responsiveness to visual stimuli.
SIMPLE CELLS: receptive fields = combination of information from RETINAL GANGLION CELLS
- -Best stimulus = a BAR of light oriented across the centers of the ganglion cells in the field
- -Might have diagonally, horizontally, or vertically oriented ganglion cells (or somewhere in between)
Discuss the role of ocular dominance columns in the organization of the primary visual cortex.
Inputs from both eyes are organized into OCULAR DOMINANCE COLUMNS in the primary visual cortex
- -The cells receiving input from a small piece of retina in one eye (an OCULAR DOMINANCE COLUMN) are next to cells receiving input from that same piece of retina in the other eye
- —Have alternating columns of cells receiving information
Discuss the concept of parallel processing of visual information and the role of the several visual association areas in the detection of various aspects of visual perception.
Visual association areas take you from a flat image of the world to COLOR, MOTION, and DEPTH
Projections from V1 (primary visual cortex) to…
- -V2, V3 (occipital lobe): depth perception
- -MT/V5 (middle temporal area): motion detection
- -V4 (occipital lobe): color vision
Describe the concept of the ventral and dorsal stream of visual information.
Dorsal pathway: motion, depth perception, relative size
–V1 -> V2, MT
Ventral pathway: object/form recognition (including color)
–V1 -> V2, V4
Describe the mechanisms involved in motion detection.
Motion:
- If you hold your eyes still, does the image move on the retina?
- Does the image stay still on the retina when you are moving your eyes?
How are ocular dominance columns established? (2)
Gradients of Eph receptors and their Ephrin ligands are important for the topographic projection from the LGN to the primary visual cortex
- -Aid in initial localization of axons from each eye into ocular dominance columns
- -Final pruning/localization is from coordinated electrical activity
SUMMARY: BOTH chemical cues (eph/ephrin) and coordinated electrical activity are necessary for the correct segregation of projections and appearance of ocular dominance columns that occurs during development of the primary visual cortex
What is strabismus?
Misalignment of the eyes
- -Eyes not aligned in the same direction and do not move together
- -A cause of amblyopia (lazy eye) -> reduced or lost vision from one eye as brain ignores input
Results form lack of correlated electrical activity from each eye
What are monocular cues used for? How do they work? (6)
Depth perception - for very long distances (>100-200 feet)
- -Previous familiarity (know how big an adult person is, so if a person is smaller, she is farther away)
- -Relative size
- -Linear perspective: parallel lines converge with distance
- -Interposition: shapes that are interrupted by other ones (man stands behind desk, can’t see legs, must be behind desk)
- -Shadows/illumination
- -Motion parallax: things closer to you move faster, things farther away move slower
What are binocular cues used for? How do they work?
Binocular cues (stereopsis): for closer things
- -Have BINOCULAR CELLS with receptive fields on both retinas
- -Maximum response for binocular cortical cell occurs for stimuli falling simultaneously on each eye’s receptive field
- -Binocular cells with receptive fields that are FARTHER away from each other (out towards the periphery of the retina) will signal that an object is CLOSE when they are stimulated
- -Binocular cells with receptive fields that are CLOSER together (near the middle of the retina) will signal that an object is FARTHER away when they are stimulated
What characteristics of color do we detect? Define constancy, hue, brightness, saturation, and gradiation.
Color constancy: the property of an object (its color)
Color perception:
- -Hue: proportion to which each cone type is stimulated
- -Brightness: total amount of stimulation of all three cone systems
- -Saturation: how much all three cone systems are stimulated to the same degree
Color gradiation: combination of hues, brightness, and saturation
How do we detect color?
Single-opponent color cells: in RETINA; have center-surround organization
- -Red on, green off
- -Green on, red off
- -Blue on, yellow off
- -Red on, blue off
Double opponent color cells: in CORTEX
- -Organized as if response is due to multiple single-opponent color cells
- -Example: best stimulus is red spot in the middle of a green surround
What is the binding problem?
Problem: how do we take all our extracted information and create a coherent image?
- -Look for similarities/cues, compare it to things we know
- -From visual field -> feature maps -> binding and merging -> master map
- —Binding and merging probably takes place in the parietal lobe
What are aperceptive visual agnosias?
Difficulty separating figures from the background
Wouldn’t be able to recognize a crossed-out figure
What is synesthesia?
A cross-modality activation of sensory systems
- -Occurs in 1/10000 people
- -Perhaps a result of excess connections between areas
Examples:
- -Hear saxophone, see neon purple snakes
- -Harmonica sounds green
- -Taste of mango sorbet makes wall look lime green
- -Tuning a violin by what it tastes like (grass if in tune, weeds/dirt if out of tune)
Diagram the visual projections from the lateral geniculate to primary visual cortex.
Form the OPTIC RADIATION
–Information from LOWER half of visual field (upper retinal quadrants) -> visual cortex ABOVE calcarine sulcus
–MEYER’S LOOP: information from UPPER half of visual field (inferior retinal quadrants) -> visual cortex BELOW calcarine sulcus
Which region of the retina is most heavily represented in the primary visual cortex?
Foveal region has disproportionately large representation in the primary visual cortex
In what orientation is the visual image map on the primary visual cortex?
Image in primary visual cortex is upside-down and reversed right to left
–Projected point to point from retina to LGN to visual cortex, but inverted/reversed based on how it hits the retina
What molecules aid in signaling for the selective decussation of nerve fibers from the nasal half of each retina at the optic chiasm?
Ephrins!
Temporal retinal ganglion cells express Eph B1, which makes them repulsed by the cells of the optic chiasm, which express Eph B2
Therefore, only axons from the nasal retina cross at the chiasm
Describe the laminar and modular organization of the primary visual cortex. Where do inputs from the LGN terminate?
- -Has six layers
- -Includes pyramidal cells and nonpyramidal cells (interneurons)
–All INPUTS terminate in LAYER 4
–Subsequent projections to layers 2/3, then 5, then 6
–OUTPUTS from layers 2-6 (2/3 -> other cortical areas; 6 -> LGN)
Discuss hypercomplex cortical cells in terms of their responsiveness to visual stimuli.
HYPERCOMPLEX CELLS: require MOTION of the appropriate stimulus in the correct direction through their receptive field
–Bar of light must be oriented correctly, only stimulating excitatory simple cell, and MOVING
Discuss complex cortical cells in terms of their responsiveness to visual stimuli.
COMPLEX CELLS: receptive fields = combination of information from SIMPLE CELLS
- -Sensitive to orientation and length of light stimulus bar
- -Best at detecting the ENDS of lines; where a bar of light stimulates their excitatory simple cell but NOT its neighboring inhibitory simple cell
What are orientation columns?
ORIENTATION COLUMNS: Neurons in primary visual cortex are highly organized according to stimulus specificity
- -Vertically, all neurons respond to same stimulus
- -Horizontally, neurons display a highly organized pattern of stimulus responsiveness
When is the critical period for development of ocular dominance periods and why is it important?
Critical period = 0-6 years
- -Period during development in which the ocular dominance columns are developed
- -Can get permanent cortical blindness if one eye is sutured shut for too long or past the end of this time
What is akinetopsia?
Motion blindness
A selective deficit in the detection of motion
